The present invention relates to a driveline system for a four wheel drive vehicle, and more particularly, to a differential device for use in such a driveline system. The use of the term "differential" does not imply the presence of conventional differential gearing, but instead, the term is used primarily because the device of the present invention replaces the typical prior art center differential. However, it should be understood that the device illustrated, described and claimed herein has uses other than as a center differential.
In many of the vehicles being produced today, the basic vehicle platform is a front wheel drive. However, in many such vehicles, especially in vans and sport utility vehicles, it is considered desirable to be able to provide four wheel drive, at least under certain operating conditions.
Typically, the various arrangements for achieving part-time four wheel drive have been complex and expensive, and in some cases, have required some sort of control scheme to achieve the four wheel drive (or rear wheel drive) in response to certain predetermined operating conditions.
In certain vehicle applications, it is acceptable to provide only front wheel drive under most operating conditions, and rear wheel drive is required only when the front drive wheels are slipping (e.g., under poor traction conditions). However, the prior art has not provided a suitable device wherein only front wheel drive is provided, with no substantial torque being transmitted to the rear wheels, until the front wheels begin to slip. For example, U.S. Pat. No. 4,562,897 discloses a viscous clutch in the driveline, between the front transaxle and the rear wheels, but requires a selector arrangement in order to choose among transmitting torque through the viscous coupling, or disconnecting the viscous coupling, or bypassing the viscous coupling. Implementation of such a selector arrangement adds substantially to the overall complexity and cost of the driveline system, and as is well known to those skilled in the art, component cost is a major factor in determining whether or not a particular system or component will be utilized on a vehicle.
It is known from U.S. Pat. No. 5,070,975 and from European Application EP 0 314 420 to use, as a center differential in a four wheel driveline, a viscous actuated ball ramp type friction clutch to transmit torque to the rear axles when there is a speed difference between the front and rear wheels. It is believed that the torque transmitting capability of the device of the cited patent and application would be somewhat limited by the fact that one of the ball ramp plates comprises the input to the viscous coupling, but despite the limited torque capability, the device of the cited patent would probably be substantially more expensive than would be acceptable for typical vehicle applications. One of the reasons for the excessive cost of the device of the cited patent is the use of a fairly conventional type of cast housing, having several different machined internal diameters, internal snap ring and O-ring grooves, and a set of internal splines.